Global discovery of erythroid translation regulators. (A) RNA-binding protein motifs enriched within the 3′UTRs of genes translationally regulated during differentiation. The 10 most enriched motifs of erythroid-expressed RBPs are shown along with enrichment statistics (see supplemental Methods). (B) Relative abundance of RBPs from (A) (rows) across 29 primary mouse cell and tissue types profiled by RNA-seq for the ENCODE consortium (columns). Heatmap displays, for each RBP in each cell or tissue type, its fractional expression level out of the total expression across all cell and tissue types examined. Asterisks mark hematopoietic cell–enriched (P < .05, Kolmogorov-Smirnov test) RBPs. (C) Rbm38 is highly induced during terminal erythroid differentiation. Primary differentiating erythroid cells were sorted (R2-R5 fractions) based on cell surface markers,⁴⁶  and mRNA expression was quantified as fragments per kilobase of exon per million mapped fragments (FPKM). (D) RBM38 is highly induced during terminal erythropoiesis in ex vivo culture. Primary erythroid precursors were differentiated in culture, and protein levels were examined by western blot analysis at the indicated time points. (E) RBM38 is cytoplasmic. Primary erythroblasts were fractionated into cytoplasmic and nuclear components, and protein levels were measured by western blot. Adj., adjusted; Anti-TBP, antibody to TATA box–binding protein. IgG, immunoglobulin G; RNaseA, ribonuclease A; Sk, skeletal.